1. Field of the Invention
The present invention relates to a system for making electrical contact between a coaxial plug or socket and a bottom plate of a microwave circuit enclosed in a shield box.
2. Description of the Prior Art
It is known for electronic circuits intended to operate at high frequencies to be implemented on a substrate, a PTFE-glass substrate, for example, in turn mounted on a conductive bottom plate, a brass bottom plate, for example. A circuit of this kind is then enclosed in an electromagnetic shield box and is electrically connected to surrounding devices by means of coaxial sockets fixed to the outside of the box, as shown in FIG. 1 which is a partial cross-section view of an electrical contact between a socket, which is an electrical connector, and an electronic circuit.
A socket connector 10 is fixed by screws 11 to a side wall 12 of an electromagnetic shielding box base. A microwave circuit comprising a conductive bottom plate 13 and a substrate 14 is fixed inside the base which includes a flange 15 all around its perimeter and cooperates with a lid 22 to form the box. The bottom plate 13 with the substrate 14 on top of it is fixed by screws (not shown) to the flange 15, which thus acts as its support.
The core or center conductor 16 of the socket 10 is partially enclosed by an insulative sheath 17, a PTFE sheath, for example, and is inserted in a hole 18 through the wall 12. After the socket 10 is screwed to the wall 12 the end of the core 16 is soldered to the substrate 14.
The main drawback of this electrical connection system is that the grounding contact to be made between the bottom plate 13 and the body of the socket 10 is made by way of the flange 15 and the interface 19 between the flange 15 and the bottom plate 13. The ground path 20 shown by a dashed line is long and requires an excellent contact at the interface 19.
Absorption of microwave energy is often observed when the grounding contact is established in this manner, especially at high frequencies, above a few GHz.
Another drawback of this connection system is that individual partitions 23 must be used if there is a requirement to separate microwave functions. Each partition 23 must then be screwed to the substrate 14 and provided on the edge facing the lid 22 with a microwave seal as there is necessarily some clearance between the lid 22 and the top of the partitions. The number of parts required to implement the microwave function is therefore large, as the partitions 23 cannot be part of the lid 22 or of the base.
To overcome this latter drawback the microwave circuit comprising the substrate on the conductive bottom plate may be fixed as shown in FIG. 2.
The microwave circuit is screwed on from underneath by the screws 25, in other words the substrate 14 bears against the flange 15. This makes it possible to reduce the number of mechanical parts used as the screening box comprises only a base 24 and the lid 22. The partitions 23 are an integral part of the base 24.
However, in this type of assembly the bottom plate 13 is no longer in electrical contact with the box, as previously. This contact can only be established indirectly, for example by plated-through holes on either side of the access microstrip line of the microwave circuit; they are then ineffective at microwave frequencies because they are too far away from this access line.
An object of the present invention is to overcome these various drawbacks.
To be more precise, one object of the invention is to provide a system for making an electrical ground connection between a conductive bottom plate of a microwave circuit contained in an electromagnetic shield box and a connection socket fixed to the exterior of said box.
Another object of the invention is to be provide a socket with which an electrical ground connection of this kind may be made.